- What's up bro? Feeling a little upset today, I see?
- Man, I don't wanna talk about it really.
- What's going on bro?
- Don't know man. I just feel like slapping a ni**a today.
Comes from the way a mongoose moves – they're known for their fast, darting movements. So, "shitting like a mongoose" might be a humorous way to describe someone rushing to the toilet with urgency.
I ate a bunch of fiber and now I've been shitting like a mongoose all day.